This invention relates to a rotational speed sensor and particularly to one incorporated within a rolling element bearing.
Anti-lock braking systems (ABS) are increasingly popular on present day motor vehicles. Such systems are provided to automatically prevent wheel lock-up during hard braking maneuvers by relieving braking pressure in such conditions so that vehicle stability and directional control can be maintained. A critical feature of an ABS is a wheel speed sensor which provides an output to an ABS controller related to wheel rotation. Many passenger cars have such a sensor for each of their four wheels. Through such inputs, the braking system controller can determine if a wheel lock-up condition has occurred or is being approached and thus control the braking system. Wheel speed sensors are also used to provide inputs for traction control systems which reduce shippage during acceleration.
Numerous designs of ABS wheel speed sensors are presently known. Such sensors generally consist of a rotating part in close proximity to a stationary part. The rotating part, or "tone ring" has features which can be sensed as they pass the stationary part. Such features are typically ferromagnetic teeth, as on a gear, or magnetic poles which have been applied to the part. The stationary part includes a transducer which can detect the passing of the features as the tone ring rotates. The detection is indicated by an electrical signal which is emitted by the transducer. The transducer may be a variable reluctance device, Hall effect device, magneto-restrictive device, or of some other construction. Generally, the transducer is a device which senses magnetic fields or changing magnetic fields. Variable reluctance transducers are referred to as "passive" sensors in that they generate a voltage without being energized by an external source. Active sensors such as a Hall effect device are energized by an externally applied voltage and provide an output responsive to the magnetic fields passing through them.
Although sensors utilizing the above described technologies have been implemented with success, designers of such systems are constantly striving to increase their reliability, reduce packaging space requirements, facilitate production, assembly and calibration, all the while seeking to reduce their cost. Of principal concern is protecting the sensor elements from contaminants and environmental exposure. Road debris such as dirt, dust and water can interfere with an unprotected sensor. In addition, dust from braking surfaces and significant temperature extremes can be applied to the sensing system unless steps are taken to protect it. A present trend in wheel speed sensor design is to integrate the sensor into a wheel bearing assembly enabling the system to be aligned and tested prior to being shipped to the vehicle manufacturer. Moreover, such an integrated configuration facilitates vehicle assembly on the assembly line.
In accordance with the present invention, an ABS wheel speed sensor system is provided in which a magnetized tone ring is provided which rotates with respect to a stationary transducer. Although these inventors have implemented this system in conjunction with an active sensor, this invention could find equal utility in a passive sensor of the type utilizing a rotating magnetized disk or other systems utilizing a rotating magnetized tone ring. A significant feature of this invention is the formation of the tone ring using a substrate coated with magnetic ink. Magnetic ink can be coated as a thin layer on the substrate and is thereafter magnetized to produce alternating magnetic poles. Preferably, the tone ring and associated transducer are within the rolling element cavity of the bearing and protected from environmental factors by the bearing seals. The ability of the sensor according to this invention to be made in a compact form enhances its ability to be sealed within the bearing cavity. It is further believed that fabrication of the tone ring which can be printed with magnetic ink instead of machined, casted or molded provides cost and design flexibility advantages. In fact, this invention would enable existing surfaces within a bearing or hub to be directly coated with magnetic ink, thereby eliminating a discrete tone ring element entirely.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.